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Hungry mums Caring for young is hard and hungry work and it seems kangaroo mothers' cope by eating more.

Researchers at the University of Melbourne studied behavioural changes in lactating female eastern grey kangaroos (Macropus giganteus) and found the hungry mums were eating one and a half times more grass than their non-reproducing counterparts but that this may come at a health cost.

Eating more and for longer periods, increased the animal's exposure to gastrointestinal parasites because they were feeding on faecal-contaminated pasture.

Study author Dr Mark Elgar says kangaroos are generally quite careful about where they forage, but when food is scarce or where requirements are high they become less discerning.

"We know mammals become more susceptible to parasites during lactation periods because the immune system is suppressed, but what we are suggesting is that on top of this, reproducing kangaroos have an enhanced likelihood of contracting disease," says Elgar.

Reorganising the diary

In general, kangaroos spend half of the day feeding, a third of the day resting and approximately one tenth of the day keeping an eye out for potential predators (vigilance).

Elgar says they had expected that in the reproducing females, the additional time spent feeding would be offset by a reduction in 'vigilance' time and that "they'd therefore be putting themselves more at risk".

"Most people think there's a trade off between obtaining food and avoiding predators," he says.

But the research revealed that vigilance time was unaffected.

Instead, resting time was reduced by 13 per cent while foraging time increased by 16 per cent. In addition, reproducing females took 20 per cent more bites than non-reproducing females.

This translated to an increased food consumption of 49 per cent.

Wild figures

"People have been interested in the costs of reproduction and how they are manifested for a very long time, but field experiments are difficult to do because you can't simply compare reproductive and non-reproductive females," says Elgar.

"Females that are reproducing may simply be in better condition; that is, have greater overall fitness."

To calculate the extent to which female kangaroos alter their behaviour during reproduction, the scientists randomly gave ten females a contraceptive implant and compared them with ten females who had been left to reproduce naturally in the Serendip sanctuary near Geelong.

Observational data was collected over three discrete seven-day periods that covered three stages of lactation: early, mid and peak; the latter being when the joeys are close to completely leaving their mother's pouch at around nine months of age.

"We have been able to prove that reproducing females in the wild alter their behaviour in direct response to the energetic demands of reproduction," says Elgar, adding that the study has broad implications for other animals including endangered species.

"This story is about kangaroos but it's also about animals more generally."

Dr Steven Cork, who runs the environmental consultancy EcoInsights in Canberra, Australia, says this research answers one of those questions about how animals adapt to being able to find energy to produce young.

"The results show how well-adapted they [kangaroos] are to their surroundings," he says.

"The alternative for an animal is to spend more time with their head down feeding in which case they'd be more prone to being eaten by something."